as you said boxes are or will be going down due to the nagra 3 switchover. so its hard to advise on buying a box for cable,the starview 6 may be worth a look as you say. that box will not unlock nagra 3,at least not in the way the boxes have untill now(do a bit more reading about starview 6 on this site or other sites).
the encryption method uptill now has been either nagravision 1 and nagravision 2,the switch over is the move to the next stage of encryption which is nagravision 3. i live in ireland and its been used here for some months now and there is no way to unlock it(as yet)but the talk is that either it will take a long time or it may never be cracked. it is now being used in the uk,so as we speak no box will unlock nagravision 3,but some areas of the uk have not switched over yet and this may take some months. so some boxes can unlock channels that are encryped in nagra 1 or 2 for a short while at least. the starview 6 you mentioned can be bought in 2 versions the cable version and the satellite version, but it will not open nagravision 3 channels. there is a thread here on this site about the box, look near the top center of your screen and you will see (search afterdawn forums)in the box to the right of that type in (starview 6 )and then click on go. you will see a list of threads and post talking about this box that might help you.
